U.S. patent number 5,597,215 [Application Number 08/452,994] was granted by the patent office on 1997-01-28 for method for increasing drive torque with controlled brake engagement.
This patent grant is currently assigned to Mercedes-Benz AG. Invention is credited to Matthias Baumann, Gerhard Fischle, Ralph Klingel, Thomas Mieslinger, Carola Pfister.
United States Patent |
5,597,215 |
Fischle , et al. |
January 28, 1997 |
Method for increasing drive torque with controlled brake
engagement
Abstract
A method for increasing the drive torque achieves the action of
a differential lock in that controlled brake engagement takes place
on one side of a driven axle provided that there is a speed
difference between the wheels on the different sides of a driven
vehicle axle which exceeds a threshold value. The regulation
engagement is started even when there are only small speed
differences between the wheels on the two sides of the driven axle.
A reference velocity of the vehicle is determined and, on the basis
of the determined reference velocity, wheel calibration and
cornering correction take place, and thereafter, the speed
difference is determined therefrom. The speed difference thus
determined is substantially based on the difference in the adhesion
on both sides of this axle.
Inventors: |
Fischle; Gerhard (Esslingen,
DE), Baumann; Matthias (Boeblingen, DE),
Klingel; Ralph (Wimsheim, DE), Mieslinger; Thomas
(Iggingen, DE), Pfister; Carola (Plodingun,
DE) |
Assignee: |
Mercedes-Benz AG
(DE)
|
Family
ID: |
6519283 |
Appl.
No.: |
08/452,994 |
Filed: |
May 30, 1995 |
Foreign Application Priority Data
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May 28, 1994 [DE] |
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44 18 773.4 |
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Current U.S.
Class: |
303/139;
180/197 |
Current CPC
Class: |
B60T
8/175 (20130101) |
Current International
Class: |
B60T
8/17 (20060101); B60T 8/175 (20060101); B60T
008/34 () |
Field of
Search: |
;303/139,140,141,142,143,144,145,188,186,190 ;180/197
;364/426.03 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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4130370A1 |
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Mar 1992 |
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DE |
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4230295A1 |
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Mar 1994 |
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DE |
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4327491A1 |
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Feb 1995 |
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DE |
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1575762 |
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Sep 1980 |
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GB |
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2181502 |
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Apr 1987 |
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GB |
|
Primary Examiner: Young; Lee W.
Attorney, Agent or Firm: Evenson, McKeown, Edwards &
Lenahan P.L.L.C.
Claims
We claim:
1. A method for increasing drive torque of a motor vehicle,
comprising the steps of
(a) measuring wheel speeds of at least driven axle wheels;
(b) subjecting the measured wheel speeds of the wheels of each
driven axle to a wheel calibration correction and a cornering
detection correction;
(c) determining a speed difference of the wheels of each driven
axle as a function of values of the wheel speeds obtained by the
wheel calibration correction and the cornering detection
correction;
(d) compensating the speed difference between wheels on one side of
each driven axle and wheels on another side of each driven
axle;
(e) determining a reference velocity representative of vehicle
velocity;
(f) producing a braking torque when a cut-in threshold value for
the speed difference is exceeded, on the wheels on the one side of
each driven axle which have a higher wheel speed only when the
wheel velocities corresponding to the wheel speeds are larger than
the reference velocity on both sides of the driven axle; and
(g) regulating the braking torque as a function of the speed
difference.
2. The method according to claim 1, wherein the reference velocity
is determined from the measured wheel speeds.
3. The method according to claim 1, wherein, with respect to each
driven axle, the wheel slip is determined, from the reference
velocity and from the wheel speeds of the wheels on the one vehicle
side of the driven axle, for the one vehicle side.
4. The method according to claim 1, wherein the braking torque
produced is reduced when the speed difference falls below a cut-off
threshold value.
5. The method according to claim 4, wherein the cutoff threshold
value is less than a cut-in threshold value.
6. The method according to claim 5, wherein the cut-off threshold
value is smaller than the cut-in threshold value by a fixed amount,
independently of the vehicle velocity.
7. The method according to claim 1, wherein the braking torque
produced is reduced whenever the reference velocity exceeds a
velocity limit value.
8. The method according to claim 1, wherein the braking torque is
produced only when the reference velocity at the instant of
fulfilling criteria for producing the braking torque, has not
exceeded a reference velocity value.
9. The method according to claim 8, wherein a velocity threshold
value is smaller than a velocity limit value for reducing the
braking torque.
10. The method according to claim 1, wherein a maximum for a
gradient when reducing the braking torque is predetermined.
11. The method according to claim 10, wherein the maximum of the
gradient is a function of the vehicle velocity, and the maximum of
the gradient is reduced when the reference velocity in
increased.
12. The method according to claim 1, wherein the braking torque is
reduced whenever a brake pedal is actuated.
13. The method according to claim 12, wherein the braking torque is
reduced with a maximum possible gradient.
14. The method according to claim 13, wherein the braking torque is
reduced whenever brake-pressure regulation is implemented by an
anti-lock system.
15. The method according to claim 1, wherein the braking torque is
reduced whenever brake-pressure regulation is implemented by an
anti-lock system.
16. The method according to claim 1, wherein, with a manual
gearbox, engine speed is measured, and the braking torque is
produced only when the engine speed exceeds a lower speed
limit.
17. The method according to claim 16, wherein the cut-in threshold
value is increased whenever, with braking torque being produced,
the engine speed falls initially below the lower speed limit.
18. The method according to claim 16, wherein the braking torque is
regulated by a subordinate control unit as a function of both the
engine speed and a nominal engine speed determined from the
reference velocity and a selected gearbox setting.
19. The method according to claim 16, wherein an average
driving-axle velocity is determined from the measured wheel speeds
on both vehicle sides of a driven axle as representative of the
engine speed.
20. The method according to claim 1, wherein braking torque is
regulated as a function of the speed difference reduced by an
auxiliary value determined from the engine speed and the braking
torque produced.
21. The method according to claim 20, wherein the auxiliary value
is determined from performance data predetermined as a function of
engine power characteristics.
22. The method according to claim 1, wherein a brake pressure is
based on braking torque in wheel brake cylinders.
23. The method according to claim 22, wherein a gradient of brake
pressure is a function of the braking torque to be produced.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is related to application Ser. No. 08/453,002
filed May 20, 1995, for "MOTOR VEHICLE TRACTION SYSTEM CONTROL
OSCILLATION DAMPING METHOD USING LOW-ADHESION WHEEL BRAKE
INTERVENTION" in the name of Gerhard FISCHLE et al.; application
Ser. No. 08/449,660 filed May 24, 1995, for "PROCEDURE FOR
CALIBRATION THE WHEEL SPEEDS FOR A MOTOR VEHICLE" in the name of
Matthias BAUMANN et al. and application Ser. No. 08/453,002, filed
May 30, 1995, for "METHOD FOR CONTROLLING VEHICLE BRAKE PRESSURE AS
A FUNCTION OF THE DEVIATION OF THE ACTUAL SLIP OF WHEELS RELATIVE
TO A DESIRED SLIP" in the name of Peter BOSCH et al.
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to an improvement in a method for
increasing drive torque with wheel speeds being detected at least
on the driven wheels, a differential being arranged on each driven
axle to compensate for a speed difference between wheels on one
vehicle side and wheels on the other vehicle side of this axle, a
braking torque being produced, when a cut-in threshold value for
the speed difference is exceeded, on the wheels on one vehicle side
of the driven axle which have a higher wheel speed, and the braking
torque being regulated as a function of the speed difference.
The periodical Auto Motor Sport, 16/86 of Feb. 8, 1986, pages 34ff,
describes the action of a differential lock in that controlled
brake engagement takes place on one side of a driven axle provided
that there is a speed difference between the wheels on the
different sides of a driven vehicle axle. Only the speed difference
between the wheels on the left and right vehicle sides of a driven
axle is taken into account. It is not possible to compare the
speeds with each other and derive different driving conditions. The
triggering threshold must, for example in order to exhibit
cornering tolerance, allow a large speed difference before the
regulated brake engagement is implemented.
An object of the present invention is to improve a
drive-torque-increasing method in that the triggering threshold
decreases, and therefore the regulation engagement can take place,
even when the speed differences between the wheels on the two sides
of the driven axle are small.
This object has been achieved according to the method of the
present invention by subjecting the determined wheel speeds of the
wheels of driven axles to a correction on the basis of a wheel
calibration and to a correction on the basis of cornering
detection, determining the speed difference of the wheels of a
driven axle on the basis of values of the wheel speeds obtained by
the wheel calibration and the cornering correction, and determining
a reference velocity representing the velocity of the vehicle, with
the braking torque being produced only when the wheel velocities
corresponding to the wheel speeds are larger than the reference
velocity on both sides of the driven axle.
On the basis of the fact that a reference velocity representing the
vehicle velocity is determined, a value for the slip of the wheel,
for which a measured value of the wheel speed exists, can be
calculated. A calibration of the wheel speeds can as a result also
then take place. By virtue of the calibration of the wheel speeds,
the differences in the wheel speed which arise on the basis of a
mutual difference in the rolling circumference of the wheels can be
computer-compensated. A wheel calibration which can be applied in
the case of the method according to the present invention is
described in unpublished DE 43 27 491 A1 (see corresponding above
cross-referenced U.S. patent application "PROCEDURE FOR CALIBRATING
THE WHEEL SPEEDS FOR A MOTOR VEHICLE" which is incorporated by
reference herein) of applicants' assignee and in this respect
explicit reference is made thereto.
Cornering correction takes place in a further step. The speed
difference of the wheels on the basis of the different cornering
radius of the wheels on the two vehicle sides is determined from
the reference velocity and from the wheel speeds as well as from
the geometrical conditions of the vehicle. It can, for example,
further be assumed that the vehicle fulfills the Ackermann
conditions during cornering. This assumption then gives the ratios
of the wheel speeds as a function of the geometrical conditions of
the vehicle.
After the wheel calibration and the cornering correction for the
wheels on both sides of a driven axle have been completed, the
speed difference is determined therefrom. The speed difference thus
determined is substantially based on the difference in the adhesion
on the two sides of this axle. It is therefore possible to feed
brake pressure into the wheel brake cylinder and therefore to
produce a braking torque on the side at which the larger wheel
speed is established even when the speed difference is small. The
braking torque thus produced produces the same effect as the
partial blocking of the differential.
The wheel speeds of non-driven wheels and the wheel speeds of
driven wheels are generally used for forming the reference
velocity. Methods for determining a reference velocity are known in
various ways, for example in the case of anti-lock systems. A
method for determining a reference velocity and for determining the
cornering-dependent wheel speeds is described in above
cross-referenced U.S. patent application "METHOD FOR CONTROLLING
VEHICLE BRAKE PRESSURE AS A FUNCTION OF THE DEVIATION OF THE ACTUAL
SLIP OF WHEELS RELATIVE TO A DESIRED SLIP", incorporated by
reference herein of applicants' assignee and in this respect
explicit reference is made thereto.
According to a further aspect of the invention, the engine speed is
advantageously taken into account when determining the braking
torque to be produced. If the engine speed data is not available,
then sufficiently accurately conclusions regarding the engine speed
can be drawn with the aid of the average driving-axle velocity. In
this situation, the first gear of the manual gearbox is assumed to
be selected with the effect that stalling of the engine due to the
regulation engagement is avoided even when the driver requires only
low engine power, contrary to what is still the case in known
systems.
Thus, brake-pressure production is, on one hand, suppressed at low
engine speeds or at low driving-axle velocity, and, on the other
hand, a limited speed difference between the wheels on the two
sides of a driven axle can intentionally be tolerated. This results
from an auxiliary value which is added to the cut-in threshold, ES,
being determined as a function of the engine speed and of the brake
pressure produced. The cut-in threshold increased by the auxiliary
value then forms the basis of the brake-pressure regulation.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects, features and advantages of the present
invention will become more readily apparent from the following
detailed description thereof when taken in conjunction with the
accompanying drawings wherein:
FIG. 1 is a flow chart for the execution of a method according to
the present invention;
FIG. 2 shows a diagram of the time profile of the method for a
specific time sequence of the speed difference between the two
vehicle sides; and
FIG. 3 is a graph of performance data for determining an auxiliary
value.
DETAILED DESCRIPTION OF THE DRAWINGS
The flow chart in FIG. 1 is based on a vehicle having a driven rear
axle and, in each case, a braked wheel on each side of the rear
axle constructed in a generally known manner. Steps 1 to 5 are used
for determining the speed difference, DLRH, between the two driven
wheels of the rear axle. The letter "w" stands for speed, the
indices or subscripts v, h, l and r stand for front, rear, left and
right, the index or subscript, a, stands for wheel calibration and
the index or subscript, d, stands for cornering correction.
In Step 1, the measured wheel speeds w.sub.vr, w.sub.vl, w.sub.hr,
w.sub.hl are fed to the slip control. In Step 2, the reference
velocity, v.sub.ref, is determined from these measured wheel
speeds. In Step 3, the wheel calibration is carried out. That is,
the wheel speeds w.sub.vra, w.sub.vla, w.sub.hra, w.sub.hla are
corrected by the wheel calibration from the measured wheel speeds
w.sub.vr, w.sub.vl, w.sub.hr, w.sub.hl. According to Step 4, the
cornering correction for these corrected wheel speeds is
determined; the cornering-corrected wheel speeds w.sub.vrad,
w.sub.vlad, w.sub.hrad, w.sub.hlad are then calculated therefrom.
In Step 5 the speed difference DLRH
is determined from the speeds determined for the driven rear
axle.
Steps 6 to 8 are used for testing whether the preconditions for
carrying out the unilateral brake engagement on the rear axle are
satisfied. In Step 6, a test is carried out as to whether the speed
difference is larger than the cut-in threshold ES. The cut-in
threshold can in this case assume values of 4 km/h and above. The
test takes place with the magnitude of the speed difference, with
the sign of the value specifying only the vehicle side requiring
engagement. If, for example, the sign speed difference, DLRH, is
negative, then engagement is required at the left vehicle side, and
if it is positive, then engagement is required at the right vehicle
side. In Step 7, a test is carried out as to whether the wheel
speeds w.sub.hlad, w.sub.hrad, are larger than a wheel speed
w.sub.ref corresponding to the reference velocity (w.sub.ref
=v.sub.ref /R, R being the radius of the wheel). In Step 8, a test
is carried out as to whether the reference velocity, v.sub.ref, is
smaller than the velocity threshold, v.sub.max1, of approximately
40 km/h. If one of these conditions is not satisfied, then the
process returns to Step 1. If all conditions are satisfied, then
brake-pressure regulation for producing a braking torque can be
implemented.
In Step 9, a test is first carried out as to whether the engine
speed, n.sub.mot, exceeds a minimum rotational speed, n.sub.min.
This step is necessary only in the case of vehicles having a manual
gearbox and is used to prevent engine stalling. Instead of the
engine speed, n.sub.mot, the rear axle velocity can also be used,
for which a minimum value is predetermined as a function of the
brake pressure produced.
In Step 10, the necessary brake-pressure gradient, PBG, is
determined. The brake-pressure gradient, PBG, is in this
illustration determined as a function of various quantities and at
least as a function of the speed difference DLRH. Further
quantities which may be used are, for example, the reference
velocity, v.sub.ref, and the engine speed, n.sub.mot. The
brake-pressure gradient, PBG, is then supplied to a conventional
type of control unit which then drives a known hydraulic unit such
that the brake pressure, PB, is produced in the brake cylinder of
the corresponding wheel in an otherwise known manner.
Steps 11 to 13 are used for testing whether the regulation can be
terminated or whether it is to be continued. In Step 11, current
values of the reference velocity, at least of the speeds of the
driven wheels w.sub.hlad, w.sub.hrad and the speed difference DLRH
are fed to the slip control. According to Step 12, a test is
carried out as to whether the reference velocity, v.sub.ref,
exceeds a velocity threshold value, v.sub.max2, of the order of 80
km/h. If this is the case, the regulation is terminated by jumping
to Step 14.
Otherwise, in Step 13, a test is carried out as to whether the
speed difference DLRH has fallen below the cut-out threshold AS. If
this is not the case, then the process returns to Step 10 and a new
value of the brake-pressure gradient to be set is determined. If
the value is less than the cut-out threshold, AS, then the process
goes to Step 14.
At Step 14, the regulation is terminated. For this purpose, the
brake pressure is progressively decreased under the control of the
control unit. Care should be taken here that the changes in the
driving behavior of the vehicle, due to the brake-pressure
reduction, must constantly be capable of being controlled by the
driver.
FIG. 2 shows the time profile of a regulation as a function of an
assumed profile of the speed difference. The difference, DLRH, in
the wheel speeds which have been subjected to the wheel calibration
and the cornering correction, is plotted as the curve k1. The speed
difference is in this case given as a velocity difference in km/h.
It is, of course, possible to convert between angular speed and
velocity using a predetermined tire diameter or using a tire
diameter determined in the wheel calibration. The threshold values
of the cut-in threshold ES, for example 4 km/h, and of the cut-off
threshold AS, for example 2 km/h, are likewise entered in the speed
profile.
The switching state, k2, represents the profile of the switching
position of control valves in the brake circuit which allow the
switching states of pressure increase, PBAUF, pressure holding,
PBK, and pressure decrease, PBAB. A braking torque is produced by
increasing the brake pressure in the wheel brake cylinder on the
vehicle side having the higher wheel speed. Pressure increase and
pressure decrease take place in the wheel brake cylinder by control
units such as PID control units, and are therefore per se control
circuits subordinate to the described regulation.
Before the instant, t1, the state of the brake system is such that
brake pressure at the wheel brake cylinders can be increased. This
is required so that, in normal driving operation, brake application
can take place without resort to a hydraulic unit. The switching
state, k2, is therefore in the pressure increase state, PBAUF.
At the instant, t1, the speed difference DLRH exceeds the cut-out
threshold, AS. In preparation for a regulation engagement, the
signal "suspend pressure decrease", Sf, is set. The switching
state, k2, changes from pressure increase, PBAUF, to pressure
holding, PBK.
At instant, t2, the speed difference, DLRH, exceeds the cut-in
threshold, ES. The start of the regulation is implemented with the
change of the cut-in signal, SE, from "0" to "1". The switching
state, k2, changes from PBK to PBAUF, and the hydraulic unit
comprising at least of solenoid valves, precharging pump and return
pump is activated to produce the desired braking torque in the
wheel brake cylinder which is arranged on the spinning driven
wheel. The brake pressure gradient, PBG, actually predetermined is
here defined by a subordinate control unit, for example, a PID
control unit.
At instant, t3, it is established that the speed difference, DLRH,
has reached a maximum. The signal, DMAX, is set. The hitherto
increased brake pressure and therefore the braking torque produced
is held constant because the switching state, k2, is set from
pressure increase, PBAUF, to pressure holding, PBK. In the case of
a short minimum holding time, e.g., approximately 500 ms, a test is
carried out as to whether the brake pressure produced is sufficient
to reduce the slip. At instant, t4, it is established that the
speed difference, DLRH, is still too large. This can happen, for
example, by analyzing the speed difference and the time derivative
of the speed difference, as soon as it has been established that
the speed difference exceeds a maximum. If the speed difference is
large and the time derivative is too small, then it is concluded
that the braking torque produced is not yet sufficient. A slip
signal, SV, is set and the switching state is again set to pressure
increase, PBAUF. The slip signal, SV, is in this case held for a
holding time of at least 500 ms, as a result of which the switching
state, k2, is fixed for at least such a period of time. The
brake-pressure regulation is simultaneously activated by the engine
speed by setting the signal, MOT.
At instant, t5, the speed difference reaches a minimum. As a
result, the signal, DMAX, is set and, consequently, the slip
signal, SV, is also reset.
At instant, t6, a maximum speed difference, DLRH, is again
detected. The signal, DMAX, is set, and the switching state, k2,
changes to pressure holding, PBK.
At instant, t9, the time derivative (with regard to its magnitude)
is large enough for an excessive decrease in the speed difference,
DLRH, to be established. A signal, ABMAX, is set. The state, k2, is
thus switched to pressure decrease, PBAB.
At instant, t10, the speed difference, DLRH, reaches a minimum. The
signals, ABMAX and DMAX, are reset, and the switching state, k2, is
switched to pressure increase, PBAUF. The switching processes at
instants t10 to t14 correspond to the switching processes t6 to
t9.
At instant, t15, it is established that the speed difference, DLRH,
has fallen below the cut-off threshold, AS. All set signals, i.e.
the signals, ANF, ABMAX, DMAX and SE, are then reset. The drive
regulation has reduced the speed difference to such an extent that
the regulation termination is initiated. The state, k2, is,
provided that this is not yet the case, switched to pressure
decrease, PBAB. The pressure decrease is controlled by a
subordinate control unit and takes place slowly enough to guarantee
that the driver does not lose control of the vehicle. In this case,
it must be taken into account that in order to compensate for the
yaw torque resulting from the unilateral brake engagement, the
driver must employ a not inconsiderable steering action, in order
for the desired driving direction to be retained.
The instant, t16, corresponds to instant, t1. It is established
that the speed difference DLRH exceeds the cut-off threshold AS.
The brake-pressure decrease is interrupted by setting the switching
state, k2, to pressure holding, PBK. At instant, t17, the cut-in
threshold, ES, is exceeded by the speed difference, DLRH, and the
regulation is reactivated by setting the cut-in signal, SE. The
switching state changes to pressure increase, PBAUF. The signal,
DMAX, is set at instant, t18, the pressure increase, PBAUF, is
terminated and the switching state pressure holding, PBK is
adopted.
Similarly, the brake-pressure regulation is activated by the
starting control unit using the signal, ANF, which is set. At
instant, t19, the speed difference, DLRH, has fallen below the
cut-off threshold, AS. All activated signals, i.e. ANF, DMAX and
SE, are reset. The regulation termination is initiated, and the
switching state, k2, is changed over to the pressure decrease
position, PBAB. The increased brake pressure and therefore the
braking torque produced are decreased by the subordinate control
unit. If the brake-pressure decrease is terminated, then the
switching state, k2, is changed back to the position pressure
increase, PBAUF. The hydraulic unit is decoupled from the brake
circuit, and normal brake application can be carried out.
FIG. 3 shows performance data for the auxiliary value, ZUS, which
is specified as a function of the reference velocity, V.sub.ref
which is a measure for a nominal value of the drive axle velocity
and therefore also a measure for a nominal value of the engine
speed, because the gearbox ratio is known and the 1st gear is
assumed to be the selected gear.
The given characteristic curves in FIG. 3 for various brake
pressures, PB, are established for a specified engine power. The
same characteristic curves are assigned to higher brake pressures,
PB, in the case of a more powerful engine, and to lower brake
pressures, PB, in the case of a less powerful engine. Intermediate
values can be calculated by interpolation.
The auxiliary value, ZUS, is determined as a function of the
reference velocity, v.sub.ref, and, if the signal, ANF, is set, is
added to the cut-in threshold ES. As a result of this addition, an
increase of the brake pressure is allowed only at higher speed
differences, DLRH. The drive axle velocity rises and, as a result
thereof, a higher engine torque can be set by the rise in the
engine speed. The auxiliary value does not influence the start of
the regulation, because the signal, ANF, can only be set if the
cut-in threshold, ES, has been satisfied beforehand and the
regulation has already been initiated. The auxiliary threshold
alters the regulation engagement because the difference, employed
as regulation quantity, between the speed difference, DLRH, and the
cut-in threshold, ES, is decreased by the auxiliary value, ZUS.
The characteristic curves represented are in each case assigned to
a range of brake pressures. The assignment to brake pressures, PB,
and the profile of the characteristic curves are in this case
defined by taking into account the brake system used and the power
characteristics of the engine obtained, for example, in driving
tests. The characteristic curve I in FIG. 3, for example, is used
below a brake pressure of, say, 20 bar. Between 20 and 65 bar of
brake pressure, PB, characteristic curve II is used; between 65 and
85 bar, characteristic curve III is used; and above 85 bar,
characteristic curve IV is used. The characteristic curves are in
this case selected such that an approximately constant engine
speed, n.sub.mot, results during a regulation.
When characteristic curve IV is used, the cut-in threshold, ES, is
increased by an auxiliary value, ZUS, at most up to a reference
velocity of 20 km/h. Above this velocity, the auxiliary value is
zero, i.e. ZUS=0. The fact that stalling of the engine is no longer
possible at higher velocities is thereby taken into account.
Although the invention has been described and illustrated in
detail, it is to be clearly understood that the same is by way of
illustration and example, and is not to be taken by way of
limitation. The spirit and scope of the present invention are to be
limited only by the terms of the appended claims.
* * * * *